Obama, speaking Monday in a conference call with reporters, said heading the delegation was “truly a dream come true” since some of her fondest memories were watching the Olympics on television. “I am beyond proud,” she said.

Her public events begin Friday with a breakfast to honor Team USA athletes at their training facility at the University of East London. Next, she’ll host a large-scale event as part of her “Let’s Move” campaign to end childhood obesity.

The event, at the U.S. ambassador’s residence, is expected to draw about 1,000 U.S. and British children who will test their athletic skills, eat healthy snacks and rub shoulders with sports greats including soccer’s David Beckham.

“That is going to be a ball,” she said.

She’ll move on to Buckingham Palace, where Queen Elizabeth will host a reception for the heads of Olympic delegations, and afterward, the first lady will attend the games’ opening ceremony.

On Saturday, she plans to visit with Samantha Cameron, the wife of British Prime Minister David Cameron, and attend Olympic competitions. On Sunday, Mrs. Obama will return to the ambassador’s residence for a brunch with members of the U.S. Olympic Committee, former Olympic athletes and Team USA supporters.

Her final public stop will be at England’s Mildenhall Air Force Base for meetings with U.S. military families stationed there.

Mrs. Obama was prominent in Chicago’s unsuccessful bid to host the 2016 games, traveling with President Barack Obama to Copenhagen in October 2009 to make the case to the International Olympic Committee, which instead chose Rio de Janeiro.

The first lady said her desire to use the Olympic spirit to inspire young people led her to challenge the U.S. Olympic Committee to get one million children active and the committee exceeded that target by planning beginner programs and sports clinics for 1.7 million.

She has designated Saturday as “Let’s Move Olympic Fun Day” so people may take part in fitness events being announced through a Web site, meetup.com. Almost 200 communities have signed up for events, according to Sam Kass, the former Chicago chef who is White House senior advisor for healthy foods initiatives.

Her visit with Mrs. Cameron see them touch on a concern for troops, military veterans and their families and discuss a U.S.-U.K.task force that shares best practices for caring for wounded warriors, including those with mental health issues, and helping troops transition into civilian life, said Ben Rhodes, a White House deputy national security adviser.

The first lady opened the call by addressing the shooting rampage in Aurora, Colo., saying she and President Barack Obama were “heartbroken.”

“Barack and I, we have seen people, and so have people across this country, we’ve seen people come together as one American family to mourn the victims of this devastating event and to support their friends and families and loved ones,” she added.

She said while the Olympics were a wonderful occasion “we are all holding the people of Aurora in our hearts, in our prayers.”

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